mudbox and motion builder are specialists in sculpting/painting and animation

the advantage mudbox has over maya for sculpting is that it's optimized for sculpting like zbrush.
the algorithms are optimized for the things it's designed to do and it handles higher amounts of polygons than maya because it does things differently
sculpt in maya, go ahead and subdivide your mesh a few time and watch as your polycountn enters the millions and watch it slow down
now do it in mudbox, watch as the polycount reaches the millions without a performance hit
why is mudbox better at this than maya? because it's not 3D

the advantage of motion builder over maya is that it's real time animation
you don't have to wait for simulations to cache and you can bump things around in real time, eg you can create a rigid body and a ragdoll and beat up the ragdoll in real time, you don't have to wait
again, it's optimized for animation, it's fast at it and it's good at it.

why would you use a concrete mixer to mix concrete instead of a household blender? because it's good at what it does and it does it better because it's built for it.

the 'necessary' tools other than maya depends on what you're into.
if it's texturing then you'd get mudbox but you proabbly wouldn't bother with motion builder unless you're going to do some serious animation

the question you might as now is why would you want to get maya when you could just get mudbox, motion builder, mental ray and nuke?
because it's big, shiny, and expensive :p

at least that's what i can tell..